1. Field of the Invention
This invention relates to the sealing of ceramic electrolyte material into electrochemical cells and is concerned more particularly with the sealing of cells having tubular ceramic electrolyte material.
2. Prior Art
Sodium sulphur cells are typical examples of cells having a solid ceramic electrolyte. Such cells utilising a molten alkali metal have to operate at elevated temperatures where the electrode materials are liquid. Seals necessary to seal these electrode materials within the cells have therefore not only to withstand highly reactive materials at these elevated temperatures, but are also subjected to temperature cycling. As a typical example, a sodium sulphur cell might contain a beta-alumina electrolyte tube closed at one end and having sodium on one face of the tube, preferably the outer face, and the sulphur/polysulphides on the other face. The cell has to be sealed to prevent escape or mixing of these materials, and a number of proposals have been made for the sealing of such cells. Compared with most metals, ceramic materials are generally weak, particularly in tensile strength and it is necessary therefore in any seal for such a cell to ensure that the ceramic material is not overstressed.
It is well-known in a sodium sulphur cell to provide a beta-alumina electrolyte tube with an alpha-alumina tubular extension at its open end so that the end portion is not ionically conductive. The alpha-alumina and beta-alumina have similar coefficients of thermal expansion and the alpha-alumina extension can be secured to the beta-alumina with a glass seal. It is known to effect the sealing making use of such an alpha-alumina element. See for example U.S. Pat. Nos. 4,074,026; 3,982,959; 3,982,957 and 3,946,751.